1. Field of the Invention
The invention is generally related to frequency synthesizers, and more particularly, is related to systems and methods for controlling the amplitude of an oscillator.
2. Related Art
Frequency synthesizers are regularly employed in communication transceivers used in numerous types of communication systems and communication technologies, such as, radio frequency communication systems. Typically, such frequency synthesizers use a phase-locked loop to precisely control the frequency of the output signal of a voltage-controlled oscillator (VCO). The frequency synthesizer typically consists of a VCO, a low pass filter, and a phase-locked loop. The VCO generates an output signal having a particular frequency. A phase detector in the phase-locked loop receives the output signal and compares the output signal to a reference frequency. Based on the comparison of the output signal to the reference frequency, the phase-locked loop generates a control signal that is provided to the low pass filter and then to the VCO. The control signal is typically received by a variable capacitor, referred to as a varactor, in the VCO. The control signal tunes the variable capacitor, thereby changing the frequency of the output signal of the VCO.
Generally, such frequency synthesizers are effective where the range of the frequency of the output signal is limited. However, in situations where the frequency of the output signal of the VCO needs to be varied over a broad frequency range, such frequency synthesizers are very problematic. For example, where frequency synthesizers are used in wireless broadband and wire-line broadband communication systems, or in multi-band devices and multi-mode devices, the limited tuning range of the varactors prevents effective tuning of the VCO and, therefore, effective channel selection. The limited tuning range of the varactors may also be problematic in situations where large fabrication process variations, temperature variations, or other variations require the output of the controllable oscillator to be varied over a broad frequency range. Additionally, varactors with a wide-tuning range are also problematic due to the fact that they are extremely sensitive to noise and interference on the control lines from the phase-locked loop. Furthermore, the non-linear characteristics of wide-tuning varactors are also problematic in the design of the loop filter in the frequency synthesizer.
Prior art frequency synthesizers are also limited in their ability to effectively control the amplitude of the output signal of the VCO as the frequency is changed. As known in the art, due to interactions between amplitude and frequency, it is desirable to maintain a constant amplitude over the entire frequency range of the VCO. Prior art frequency synthesizers include a peak detector, a low pass filter, an amplifier, and a noise filter. The peak detector is used to determine the amplitude of the output signal of the VCO and generate a control signal. The control signal is filtered by the low pass filter and amplified by the amplifier. The filtered and amplified signal is received by the noise filter. The noise filter is needed to reject bias noise before providing the resulting signal to the VCO. The VCO uses the resulting signal to change the bias current, and thereby control the amplitude of the output signal. However, such amplitude control methods are extremely slow and unstable. The noise filter is essentially an RC circuit. Because the control signal for changing the amplitude passes through the capacitor, there is significant delay in controlling the amplitude. Thus, there is a need in the industry to address these deficiencies and inadequacies.
The invention provides a system for controlling the amplitude of the output signal of a controllable oscillator in a frequency synthesizer. The invention provides a circuit having a controllable oscillator and an amplitude control circuit. The controllable oscillator is configured to generate an output signal having a predefined frequency and a predefined amplitude. The controllable oscillator is also configured with a plurality of operational states that are controlled by the amplitude control circuit. Each operational state of the controllable oscillator defines a particular current bias associated with a distinct amplitude of the output signal of the controllable oscillator. The amplitude control circuit receives the output signal of the controllable oscillator and determines the amplitude. When the amplitude of the output signal of the controllable oscillator is less than the predefined amplitude, the amplitude control circuit provides a control signal to the controllable oscillator. The control signal is configured to change the controllable oscillator to the operational state corresponding to the distinct amplitude that best approximates the predefined amplitude.
The invention also provides related methods of operation and computer readable media. Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.